A motion transmission system in a bicycle comprises a chain extending between toothed wheels associated with the axle of the pedal cranks and with the hub of the rear wheel. When there is an assembly of toothed wheels comprising more than one toothed wheel at at least one of the axle of the pedal cranks and the hub of the rear wheel, and the motion transmission system is therefore provided with a gearshift, a front derailleur and/or a rear derailleur are provided for. In high-performance bicycles, and particularly in bicycles used in sports competitions, characteristics that are very important to the cyclist are the speed and precision with which gearshifting can be performed. Particularly for this reason, so-called electronic gearshifts, or more specifically electronically servo-assisted gearshifts, have become common.
In case of an electronically servo-assisted gearshift, each derailleur comprises a guide element—also known as chain guide or cage or, in case of a rear derailleur, rocker arm—movable to displace the chain among the toothed wheels in order to change the gear ratio, and an electromechanical actuator to displace the chain guide. The actuator in turn typically comprises a motor, typically an electric motor, coupled with the chain guide through a linkage such as an articulated parallelogram, a rack system or a worm screw system, as well as a sensor or transducer of the position, speed, acceleration and/or direction of rotation of the rotor or of any moving part downstream of the rotor, down to the chain guide itself. It is worthwhile noting that slightly different terminology from that used in this context is also in use.
Control electronics changes the gear ratio automatically, for example based on one or more detected variables, such as the travel speed, the cadence of rotation of the pedal cranks, the torque applied to the pedal cranks, the slope of the travel terrain, the heart rate of the cyclist and similar, and/or the gear ratio is changed based on commands manually input by the cyclist through suitable control members, for example levers and/or buttons.
A device or unit for controlling the front derailleur and a device or unit for controlling the rear derailleur—or only one of the two in case of simpler gearshifts—are mounted so as to be easily manoeuvred by the cyclist, normally on the handlebars, close to the handgrips thereof where the brake lever is also located for controlling the brake of the front and rear wheel, respectively. Control devices that allow to drive both a derailleur in the two directions and a brake are commonly called integrated controls.
It is in general known to drive the derailleur of a bicycle gearshift with reference to values of a table of command values, each correlated to a position of the derailleur in which engagement of the chain with a specific toothed wheel occurs or there is. In other words, the control electronics or controller uses the table of command values to obtain the value that a variable of the derailleur must take to position the chain in engagement with the desired toothed wheel. Such a value can be a differential value with respect to the adjacent toothed wheel, or it can be an absolute value with respect to a reference, for example with respect to a reference toothed wheel or to an end of stroke condition or a condition of lack of excitation of the motor.
From the point of view of magnitude, an actuator command value of the table of values can be, for example, the distance travelled by a mobile point taken as a reference on the derailleur, the number of steps or revolutions that the motor should be made to perform, a length of excitation time of the motor, the value of a supply voltage of a motor having an excursion proportional to the voltage, or furthermore it can be the value emitted by the sensor or transducer associated with the motor, a numerical value stored in a register and representative of one of the aforementioned quantities, etc.
In particular, the motors of actuators can be driven for a number of steps or for a length of excitation time or with a voltage that are appropriate for each upward or downward gearshifting and then stopped automatically, while the sensors are used to provide a feedback signal to the control electronics so that it can possibly take care of actuating the motors of the actuators again in case the intended position has not been reached, namely in case the aforementioned variable of the derailleur has not taken the table value. This may, for example, be due to the fact that the resistance torque offered by the derailleur, which is to a certain extent dependent on how the cyclist is pedalling, was too high, greater than the maximum torque able to be delivered by the motors through the linkage.
The values of said table of command values are nominal values, set in the factory, which take the number of toothed wheels in the derailleur (front or rear) and the respective thicknesses and pitches into account. Typically, such nominal values provide that, in the absence of the actuator driving signal, namely with command value at zero, the chain is in engagement with the toothed wheel having the smallest diameter, although, as can be seen from the aforementioned examples, this condition is not necessary.
Although with electronically servo-assisted gearshifts it has been possible to improve precision and speed of gearshifting, there is still a need to improve these performances, in order to be able to assist the cyclist in achieving better and better results in competition and protect the mechanics.
This requirement is clearly of increasing importance the more the gearshift is intended to be used in high-level cycling competitions.
In order to obtain high precision of control of known bicycle gearshifts, an initial adjustment of the bicycle is carried out in order to optimize the tensioning of the chain depending on the configuration and structure of the frame and the assembly of toothed wheels, both the front ones (crowns) associated with the axle of the pedal cranks, and the rear ones (sprockets) associated with the rear wheel.
It is possible to adapt the command values of the table so that they accurately correspond to the electromechanical components of the specific gearshift, in particular to the pitches of the toothed wheels and/or to the mutual position of the elements of the motor or of the linkage taken as fixed reference and as mobile reference, as well as possibly to the progress of the actuation voltage of the motor, to the speed, acceleration and/or direction of rotation of the motor, etc.
Moreover, the initial adjustment typically acts on at least one spring provided for in the transmission, possibly setting a preload on it, in order to keep the correct tension of the transmission chain in the different travel configurations.
In some gearshifts, used particularly for road bicycles, there are two chain tensioning springs counteracting to determine the setup of the chain guide when engaged with the transmission chain. This allows greater versatility of the system, providing it with high elasticity. Other gearshifts, on the other hand, can provide for only one chain tensioning spring.